You look in the dictionary. Information on the structure and
storage characteristics for just about any Oracle object can be
found in the rdbms data dictionary. To find a list of all the
dictionary tables available to you, you can query the view dictionary
which also includes the dynamic performance views, v$, tables:

There are generally three versions of every dictionary table,
which are actually views, of the form: all_tables, dba_tables,
and user_tables.

USER_x provides information on all 'x' that you own
All_x provides information on all 'x' that you have privilege on
DBA_x provides information on all 'x' that exist in database

And x is often an Oracle object type such as tables, indexes,
constraints, views, synonyms, and so on. Normally the only
difference between the columns shown by each view is that the
owner column is added to the all and dba versions since the user
version would not need the owner column.

Some of the more commonly accessed dictionary views are:

all_tables all the tables the current user can access
all_tab_columns all the columns by table for the tables the current user can access
all_constraints all constraints (PK, FK, Unique, check, and Not Null) on tables accessible by current user
all_indexes all the indexes on the tables the current user can access
all_ind_columns all the columns in the indexes on the tables the current user can access
all_objects all objects the current user has access to
all_source all source for stored procedures, functions, and packages the current user has execute on
all_synonyms all synonyms accessible to the current user
all_triggers all triggers owned by or on tables owned by the current user, not on all accessible tables
all_users all the users visible to the current user
all_views all views accessible by the current user (includes source)

There are more than 300 dictionary views available to you.
There are views for privileges granted, auditing, histograms,
partitioned objects, and nearly everything else in Oracle. It
takes time and you need to learn about them all.

Further reading: For version 7.3 up see the Oracle {version#}
Reference manual for a listing and descriptions of the Dictionary
views and the dynamic performance tables.